Data networks are formed from a number of network nodes and allow communication between a number of subscribers. Communication here means the transmission of data between the subscribers. The data to be transmitted in this case is sent as data telegrams, which means that the data is packed into one or more packets and sent in this form over the data network to the appropriate recipient. The term data packet is thus used. The term transmission of data is used in this document fully synonymously with the above-mentioned transmission of data telegrams or data packets.
For networking in switchable high-performance data networks for example, especially Ethernet, the subscribers are interlinked via coupling nodes. Each coupling node can be connected to more than two subscribers and can also be a subscriber itself. Subscribers are for example computers, Programmable Logic Controllers (PLC) or other machines which exchange electronic data with other machines and especially process it.
A method of transmitting data packets is known from EP 0 866 579 A1 in which the data packet is transferred from the transmit station to the receive station, without having an individual identification.
In distributed automation systems, for example in the area of drive technology, specific data must arrive at specific times at the intended subscribers and must be processed by the recipients. This is referred to as realtime-critical data or realtime-critical data traffic since if the data does not arrive at its intended destination at the right time this can produce undesired results at the subscriber.
The use of a synchronous clocked communication system with equidistant characteristics in an automation system is also known from the prior art. This is taken to mean a system consisting of at least two subscribers that are linked via a data network for the purposes of mutual exchange of data or mutual transmission of data. In this case data is exchanged cyclically in equidistant communication cycles which are specified by the communication clock used by the system. Subscribers are for example central automation devices, e.g., Programmable Logic Controllers (PLC) or other control units, computers or machines which exchange electronic data with other machines and process data, especially from other machines and peripheral devices, e.g. input/output modules, drives, actors, sensors. In this document control units are taken to mean closed-loop controllers or control units of all types. Typical examples of communication systems used for data transmission are bus systems such as Field Bus, Profibus, Ethernet, Industrial Ethernet, FireWire or also PC-internal bus systems (PCI), etc. In such systems data telegrams are fed into the data network at fixed points for transmission by a subscriber.
Automation components (e.g. controllers, drives, . . . ) nowadays often have an interface to a cyclically clocked communication system. A run level of the automation components (fast-cycle) (e.g. positional control in a controller, torque control of the drive) is synchronized to the communication cycle. This defines the communication timing. Other lower-performance algorithms (slow-cycle) (e.g. temperature controllers) of the automation components can also only communicate via this communication clock with other components (e.g. binary switches for fans, pumps, . . . ), although a slower cycle would be adequate. Using only one communication clock for transmission of all information in the system produces high demands on the bandwidth of the transmission link and on the communications arrangement of the components.
These high demands cannot always be fulfilled and the result can be transmission errors. Possible transmission errors are:                non-arrival or late arrival of a data telegram;        overflow of a data buffer in a coupling node;        incorrect length of a data telegram;        a physical CRC (Cyclic Redundancy Check) error.        
With data networks known from the prior art an incorrect data telegram can trigger an error reaction at all data network nodes via which it is forwarded. Error diagnosis is only possible with the telegrams intended for the purpose. This leads to an additional load on the data network, and makes error diagnosis more difficult and causes delays in diagnosis.